1. Field of the Disclosure
The present disclosure relates generally to electrophotographic imaging devices such as a printer or multifunction device having printing capability, and more particularly to a shading system for a laser scanning unit.
2. Description of the Related Art
In various imaging devices which utilize light to form images, optical scanning systems are typically incorporated to scan laser beams from one or more light sources onto a target image plane surface. In an electrophotographic imaging device, for example, the image plane surface is typically a photosensitive member. Generally, laser beams are swept across the image plane surface by a scanning mirror to form light spots upon the image plane surface along a scan line direction. Commonly used scanning mirrors include rotating polygon mirrors which scan laser beams in one direction.
A polygon mirror can have either an under-filled or over-filled facet design. In an under-filled design, the facet length is significantly wider than the incident laser beam width such that the beam footprint on a facet never crosses over the edges of the facet from start to end of a scan line operation. On the other hand, an over-filled design has a facet length that is narrower than the incident laser beam such that the beam footprint on a facet completely fills the facet and extends beyond its edges over the duration of a scan line operation. In this case, the width of the laser beam after it is reflected by the polygon mirror is determined by the size of the polygon facet.
Generally, in order to have a decent optical performance particularly on laser spot size, the width of a laser beam striking a polygon facet must be at least some requisite value. By comparison, for a given number of polygon facets, the under-filled design would require a larger polygon diameter since size of a facet would have to be wider than the requisite beam width, while the over-filled design would require a smaller polygon diameter since length of a facet only needs to be at least the same as the requisite beam width. In the under-filled design with relatively larger facets, the entire beam, in the form of a focused line segment, is projected onto a facet and reflected downstream to the photosensitive member as the polygon mirror rotates. The power profile of this line segment is typically Gaussian in nature and the resultant spot at the photosensitive member is composed of the entire Gaussian profile since the entire line segment is received and reflected by the facet. Thus, spot power variation across scan lines is generally consistent from unit to unit which allows for the same set of characterization data to be used for all units of the same type.
However, in the over-filled design with relatively smaller facets and smaller diameter polygon mirrors, the line segment overfills a facet. As the polygon mirror rotates, different sections of the Gaussian profile is reflected downstream and the focused spot at the photosensitive member has relatively higher variation in power, often referred to as power rolloff, as the focused spot moves across the scan line. Unit to unit variation typically exists because of accumulated tolerances brought about by difficulty of having precise optical alignment of optical components before the polygon mirror. When unit to unit variation exists, projection of the line segment onto a facet may vary and the way in which the polygon mirror reflects sections of the Gaussian profile onto the photosensitive member may also vary from a unit to unit perspective. As a result, spot power variation or power rolloff across the scan line may vary for different identical units even if they are characterized with the same set of characterization data, and print density darkness variation may become objectionable especially when the variation is relatively large.